Association between infant mortality and community social vulnerability in the USA: a cross-sectional study.

Introduction: Infant mortality rates in the USA demonstrate significant disparities among marginalised racial and ethnic groups. The Centers for Disease Control and Prevention and Agency for Toxic Substances and Disease Registry Social Vulnerability Index (SVI) represents a novel metric to identify patients with increased social vulnerability who may be at risk for worse health outcomes. The objective of this study was to examine the association between county-level SVI and infant mortality rates.

Methods: We conducted a retrospective cross-sectional study using the 2018 National Center for Health Statistics Restricted Vitals Statistics birth cohort linked files. We used multivariable negative binomial regression with the county-level count of infant mortality as the dependent variable and SVI as the independent variable and adjusted for maternal factors (proportion of women in each county with each of the following characteristics: aged <19 years, unmarried, inadequate prenatal care, maternal smoking during pregnancy and ≥2 maternal risk factors).

Results: There were 3 791 712 infants in the study. Increased social vulnerability, measured by higher county-level SVI score, was associated with increasing infant mortality; quintile 2: adjusted incidence rate ratio (aIRR) 1.07 (95% CI 1.01 to 1.13); quintile 3: 1.11 (1.02 to 1.20); quintile 4: 1.14 (1.03 to 1.28); quintile 5: 1.21 (1.05 to 1.40). Being unmarried (aIRR 1.09 (1.01 to 1.13)), smoking (aIRR 1.05 (1.01 to 1.10)) and having inadequate prenatal care (aIRR 1.09 (1.01 to 1.16)) were also associated with increased infant mortality.

Conclusions: County-level infant mortality was associated with increasing levels of social vulnerability in a stepwise fashion.